Identification documents, such as identity cards, passports, drivers licenses etc. are sometimes tampered with by exchanging the photograph and/or by its altering the information contained in the documents. Great efforts are therefore undertaken to render such identification documents forgery-resistant.
British Pat. No. 888,359 discloses applying a cover sheet and a back sheet of polyester film to identification cards in which a photograph is mounted on a paper core sheet. The polyester film is provided with a coating of thermoplastic adhesive, by which it is then bonded to the paper core. The adhesion of this film is, however, not very high, and it is consequently still relatively easy to peel the cover film off the support without damaging the support.
To improve adhesion, it has been proposed in U.S. Pat. No. 3,413,171 to coat the paper support with a thermoplastic adhesive layer of the same kind. But also in this case, it is apparent that the thermoplastic bonding material adheres poorly to the surface of the photograph, and the protective covering still comes off the support or can be removed by unauthorized persons without leaving any noticeable trace on the photograph.
German Auslegeschrift 2,736,852 discloses protective coverings for identification cards, in which the thermoplastic adhesive layer of the cover sheet is provided with an additional coating to improve bonding, the coating being applied in the form of a pattern. There is, however, no indication as to the adhesion of the protective covering, in particular under thermal stress or upon treatment with solvents. It is also a disadvantage of this film that it is technically difficult and expensive to manufacture. The protective covering is apparently intended for bonding only to photographs which are still moist.
U.S. Pat. No. 4,093,489 describes protective coatings for photographs, which consist of thermosetting catalyst-containing melamine/formaldehyde condensation products. The coatings are cured at high pressure for a period ranging between 3 and 10 minutes, depending on the circumstances of each particular case. This process has the disadvantage that, due to the long pressing times required, it is unsuitable for a continuous operation in which the material is continuously taken off a roll.
U.S. Pat. No. 4,115,618 discloses protective sheets for identification cards which comprise a cellulose triacetate layer which is hydrolized on one surface and, bonded thereto, a layer consisting of a mixture of polyvinyl alcohol and polyvinyl acetate. A drawback of this film is that lamination must be effected while the photograph is moist.
A film which is intended to be used as a protective covering in the manufacture of forgery-resistant identification documents must meet numerous requirements. The laminated protective film should withstand any attempt to detach it from the support at room temperature or even upon heat treatment. It should be readily weldable and resistant to delamination, even when treated with conventional organic solvents. Foremost, however, it must adhere so firmly to the gelatin layer of the photograph that it cannot be detached under the above-specified conditions without destroying the photograph. In addition, the film must be scratch-resistant, fast to light, free of yellowing and ageing-resistant. If the information carriers are contained in the identification card in the form of an embossing, the protective film must additionally be capable of being embossed satisfactorily, i.e. quickly and with sharp outlines, and embossing must not lead to cracks or fissures.
None of the coatings described hereinabove meet all these requirements simultaneously.